Apparent inhibition of photoredox reactions of magnesium octaethylporphyrin at the lipid bilayer-water interface by neutral quinones

Krakover, T.; Ilani, Asher; Mauzerall, D.

doi:10.1016/s0006-3495(81)84776-5

Cited by 10 publications

(4 citation statements)

References 5 publications

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“…If they were mostly dissolved in the bulk lipid phase the value of k' should have been much smaller than k. This is also in accord with the observation of the high a value for DCBQ (see Table I), which nominally corresponds to a bulk partition coefficient between hydrophobic solvent and water of more than 1,000, whereas the actual number for benzene-type molecules is only in the range of 10-30 (Katz and Diamond, 1974). The notion that quinones are mostly adsorbed at the membrane-water interface was already suggested by a completely independent observation regarding photodriven electron transport from porphyrin to quinone in lipid bilayer membranes (Krakover et al, 1981).…”

Section: Discussionmentioning

confidence: 86%

Diffusion- and reaction rate-limited redox processes mediated by quinones through bilayer lipid membranes

Ilani¹,

Krakover²

1987

Biophysical Journal

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The mediation of redox reactions through bilayer lipid membranes was studied. With an appropriate choice of electron acceptors the redox process can be limited either by the chemical reaction rate between the mediator and the reactants or by the shuttle frequency of the mediator through the membrane. Both modes were demonstrated for redox reactions mediated by 2,6 dichlorobenzoquinone (DCBQ) and by alpha-tocopherol with ascorbate entrapped inside vesicles using ferricyanide (a mild oxidant) or hexachloroiridate (a strong oxidant) in the external solution. The redox processes were reaction rate-limited and diffusion-limited for ferricyanide and hexachloroiridate, respectively. The kinetics of the redox processes in the diffusion- and the reaction rate-limited modes allows the determination of the shuttle frequencies and of the interfacial reaction rates of the mediators, respectively. The shuttle frequencies of DCBQ and alpha-tocopherol were approximately 8 and 0.08 s-1, respectively, in L-alpha-dipalmitoyl phosphatidylcholine (DPPC) cholesterol vesicles at 25 degrees C. Interfacial reaction rates between the mediators and ferricyanide were about two- and tenfold lower compared with bulk reaction rates for DCBQ (water) and tocopherol (50% ethanol solution), respectively, i.e., tocopherol is relatively less accessible to aqueous oxidants at the membrane interface. Tocopherol and oxidized tocopherol are reversible hydrophobic redox couples that interact very rapidly with strong oxidants. In both modes of mediation DCBQ was more effective than alpha-tocopherol.

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Section: Discussionmentioning

confidence: 86%

Diffusion- and reaction rate-limited redox processes mediated by quinones through bilayer lipid membranes

Ilani¹,

Krakover²

1987

Biophysical Journal

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“…In the absence of other acceptors or donors, no photovoltage is seen because of the vectorial nature of the interfacial electron transfer, and the presence of equal concentrations of 02 on both sides of the membrane. It is thus similar to neutral quinones (Krakover et al, 1981). The fraction of ions formed from 02 quenching can be readily estimated by adding sufficient donor to one side such that the lifetime of the pigment cation is reduced to <1 ,gs.…”

Section: Discussionmentioning

confidence: 94%

The effect of oxygen on the amplitude of photodriven electron transfer across the lipid bilayer-water interface

Ilani¹,

Liu²,

Mauzerall³

1985

Biophysical Journal

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The surprisingly small effect of oxygen on photoelectron transfer in pigmented lipid bilayers is traced to a short lifetime of the excited states. Decreasing the oxygen concentration by greater than 100-fold decreases the half saturating concentration of acceptor by only threefold and has no effect on the maximum photovoltage observed at acceptor saturation. This holds true for both magnesium octaethylporphyrin and chlorophyll with both ferricyanide and methyl viologen as acceptors. Since oxygen quenches excited states at near the encounter limit, the lifetime of reactive state must be short, less than 100 ns. About 100-fold higher concentrations of acceptor are required to quench the fluorescence (in liposomes) than to saturate the photoeffect. Thus the reactive state is most likely the triplet. The short life of the excited state is caused by concentration quenching, i.e., their reaction with ground state molecules. The increase of photovoltage with increasing pigment concentration shows that this quenching in a condensed form of the pigment produces ions that lead to the observed photovoltage by interfacial reaction of the anion with acceptor.

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“…For ferricyanide, though, this is not necessarily the case. In fact, the increase in rate of ferricyanide photoreduction shown in Fig, 3 following addition of DQ, a transbilayer mediator (Krakover et al, 1981;' Ilani and Krakover, 1987), implies that transbilayer movement is rate limiting for ferricyanide reduction. Therefore, in the absence of DQ the rate limiting step for ferricyanide reduction is transbilayer movement of the porphyrin, whereas the interfacial reaction is rate limiting for MV2+ reduction.…”

Section: Discussionmentioning

confidence: 99%

“…Photoinitiated electron transfer reactions in lipid membranes, both planar and vesicular, have been studied extensively and are becoming well characterized as a model for biological electron transfer (see Hong, 1976 for review; Ilani and Mauzerall, 1981;Liu and Mauzerall, 1985;Ford and Tollin, 1984;Woodle and Mauzerall, 1986;Woodle et al, 1987). Bilayer lipid membranes have also proved useful as an organized system for separation of aqueous reactants on either side with bilayer bound components mediating the reaction (see Fendler 1984 for review;Ford ef al., 1979;Krakover et al, 1981;Matsuo et al, 1980;Runquist and Loach, 1981;Tunuli and Fendler, 1981;Bienvenue et al, 1984;Ilani and Krakover, 1987). The photodependent processes can be either photoinitiated or photodriven.…”

Section: Introductionmentioning

confidence: 99%

Photoinduced Electron Transfer Across Lipid Bilayers Containing Magnesium Octaethylporphyrin

Ilani

Woodle

Mauzerall

1989

Photochem & Photobiology

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Both photoinitiated (thermodynamically downhill) and photodriven (thermodynamically uphill) electron transfer reactions across lipid bilayers are sensitized by magnesium octaethyl porphyrin (MgOEP). It is shown that the reaction mechanism is via reduction of photoexcited MgOEP at the reducing (ascorbate) side of the bilayer and the charge carrier is likely the neutral protonated MgOEP anion. The MgOEP cation (or its neutral form) does not contribute to charge passage across the bilayer even though it is readily formed at the acceptor (ferricyanide or methyl viologen) side of the membrane. Photoelectric measurements on planar bilayers show that the time constant for reduction of excited MgOEP is about 10 microseconds with 10 mM ascorbate. The membrane transport of the mediator appears to be rate limiting when the reaction is photoinitiated and the interfacial reaction appears to be limiting when the reaction is photodriven. The quantum yield of the process is about 0.1 in the latter case and about 0.02 in the former. The former yield is increased to about 0.15 in the presence of a redox mediator, duroquinone. In these systems, the magnesium porphyrin is both sensitizer and trans membrane redox mediator.

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Apparent inhibition of photoredox reactions of magnesium octaethylporphyrin at the lipid bilayer-water interface by neutral quinones

Cited by 10 publications

References 5 publications

Diffusion- and reaction rate-limited redox processes mediated by quinones through bilayer lipid membranes

Diffusion- and reaction rate-limited redox processes mediated by quinones through bilayer lipid membranes

The effect of oxygen on the amplitude of photodriven electron transfer across the lipid bilayer-water interface

Photoinduced Electron Transfer Across Lipid Bilayers Containing Magnesium Octaethylporphyrin

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